1. Field of the Invention
The present invention relates to an AF (auto focusing) control apparatus for a zoom lens for controlling movable lens groups serving as a zoom lens and a focusing lens.
2. Description of the Related Art
The publication of Japanese Patent No. 2548307 discloses a zoom lens system having a plurality of lens groups. In this zoom lens system, only the rearmost lens group positioned closest to the image side operates as a focusing lens group. However, since the focusing sensitivity of the rearmost lens group is almost constant regardless of the focal length of the zoom lens system, the moving distance of the rearmost lens group during focusing is larger at the telephoto side than at the wide-angle side. Therefore, in such an AF mechanism, the AF operation at the telephoto side requires a large amount of time. Also, in order to shorten the minimum photographing distance as much as possible, a large moving distance of the rearmost lens group is required. However, the moving distance of the rearmost lens group cannot be sufficiently increased since interference with the lens group provide in front of the rearmost lens group must be avoided. Therefore, the minimum photographing distance cannot be shortened very much.
In this regard, an AF control apparatus has been developed in which two lens groups in a zoom lens system having a plurality of lens groups serve as a focusing lens. The two lens groups are moved together along an optical axis so as to reduce the moving distance of the two focusing lens groups during an AF operation and to shorten the AF operation time and the minimum photographing distance particularly in a telephoto side (for example, Japanese Patent Laid-Open Publication No. Hei 7-120678).
Such an AF control apparatus with two movable lens groups may be configured through, for example, providing pulse motors for moving the two focusing lens groups in a direction of an optical axis, and driving the pulse motors according to data stored in a focusing table.
In such an AF control apparatus, a simpler control system can be configured through controlling the pulse motors during zooming by use of a focusing table.
However, in the case when the image magnification is changed stepwise from a wide-angle side to a telephoto side, a zooming operation carried out via focusing table data sometimes results in a reversed phenomenon in which an image magnification becomes larger than that of the previous zoom step, or an unchanged phenomenon in which a magnification does not change in an intermediate zoom step. If images are continuously displayed on the monitor of a digital camera under this situation, the user may notice such unnatural zooming. Such a reversed phenomenon or unchanged phenomenon also occurs when magnification is varied stepwise from a telephoto side to a wide-angle side.